1. Field of the Invention
The present invention relates to telephony, and more specifically to a method and device for reducing coupling between neighboring interface circuits connected to different subscriber lines that convey digital information in analog form.
2. Description of Related Art
Conventionally, baseband transcoding includes two steps: a coding step in which a predetermined symbol is associated with a sequence of successive digital information items, and a shaping step in which an elementary analog pulse of predetermined duration is associated with each symbol after digital-to-analog conversion. The resulting pulses form an analog signal that is delivered to the line transformer arranged at the end of the subscriber line. Among the codes typically used is a first type known by the name xe2x80x9c2B1Qxe2x80x9d and defined in the standards ETSI ETR 080 (European standard) and ANSI T1.601 (American standard), and another type known by the name xe2x80x9c4B3Txe2x80x9d and defined in the standards ETSI ETR 080 (European standard) and FTZ 1 TR 220 (German standard).
One conventional application of such baseband transcoding is to the transmission of digital information within the context of a digital network of universal character, such as the integrated services digital network (ISDN) defined in the standards ETSI ETR 080 (European standard) and ANSI T1.601 (American standard). The three main characteristics of ISDN are: (1) the capability of simultaneously employing telephone, telematic, and even video-communication services from one access; (2) subscriber-to-subscriber integral digital transmission with digital connections in the various switching centers; and (3) signaling in message mode only between the various functional entities (e.g., terminals, switches, and servers).
The architecture of the local network attaching users to an ISDN network includes: on the subscriber side, a network digital terminal (TNR) to which the various user terminals are attached, and on the switch side, a line terminal (TL) that provides substantially the same functions as the TNR terminal. Between the TNR terminal and the TL terminal there is a subscriber line (U interface) that consists of a pair of copper cables whose characteristics (e.g., maximum length, useful bit rate, and error rate) are standardized. The useful bit rate with ISDN is 144 Kbits/s, which is used by two xe2x80x9cBxe2x80x9d channels operating at 64 Kbits/s for transporting useful information and a xe2x80x9cDxe2x80x9d channel operating at 16 Kbits/s for transporting check bits. Thus, the subscriber line alone provides the so-called xe2x80x9cnarrow bandxe2x80x9d ISDN connection that allows access at the basic bit rate (2B+D) of 144 Kbits/s. By using codes such as the 4B3T code or the 2B1Q code, it becomes possible to reduce the spectral band used for transmission.
The U interface circuit is a transmitter/receiver circuit that is located at the TNR terminal and also at the TL terminal. The U interface allows bi-directional transmission of digital data over a single telephone pair, with transmission being carried out in xe2x80x9cfull-duplexxe2x80x9d mode (i.e., the data transmitted and the data to be received are simultaneously on the telephone line). On reception, the data must be decoupled by echo cancellation circuitry that subtracts the signal that has just been transmitted from the received signal. Currently, it is possible to use a single mother card having up to sixteen identical U interface circuits associated with sixteen line transformers in a TL line terminal. However, if the line transformers are too close together, nuisance couplings between U interface circuits occur when a transmitter of one U interface circuit is transmitting while a receiver of another U interface circuit is receiving. As a consequence, disturbances are caused in the received signal on the reception path of a U interface circuit by the transmission paths of the immediately adjacent U interface circuits. This problem can be solved by physically distancing the transformers from one another, but this limits the number of subscriber lines that can be processed at a line terminal.
Further, new technology has recently allowed novel U interface circuits that can incorporate up to four identical transmission paths. However, in such circuits, nuisance coupling between paths is caused by the presence of a common supply as well as of other common elements. The coupling can be limited by designing such U interface circuits with separate power supplies and a minimum number of (or even zero) common circuits. However, such a solution increases the area of the U interface circuits.
In view of these drawbacks, it is an object of the present invention to remove the above-mentioned drawbacks and to provide a new solution that reduces disturbing effects of coupling between neighboring transmission/reception devices that are connected to two subscriber lines. The effect of coupling between two neighboring U interface circuits is estimated, and the estimated coupling signal is removed from the signal received by a U interface circuit.
One embodiment of the present invention provides a method for reducing disturbing effects of coupling between a first transmission/reception device and a second transmission/reception device that are each connected to a subscriber line that conveys streams of symbols in a temporally synchronous manner. According to the method, a signal received on a reception path of the first device is delayed by a delay equal to p times the transmission period. A coupling signal relating to a transmission path of a second device and the reception path of the first device is estimated based on a signal transmitted over the transmission path of the second device, and the delayed signal is ridded of the estimated coupling signal. In one preferred method, the coupling signal is estimated based on successive sums of n coefficients of a coupling estimation adaptive filter that are determined based on the signal transmitted over the transmission path of the second device.
Another embodiment of the present invention provides a device for transmitting/receiving a signal. The device includes a transmission path for transmitting a transmission signal, a reception path for receiving a reception signal, and a memory coupled to the reception path for temporarily storing p symbols. Additionally, a coupling estimation block is provided that includes an adaptive filter with n (n greater than p) coupling coefficients and an estimation circuit that receives an ancillary transmission signal of another transmission/reception device. The estimation circuit and adaptive filter successively determine the n coupling coefficients as a function of the ancillary transmission signal and sum the coupling coefficients successively so as to deliver a coupling signal that is representative of estimated coupling between the reception path and the transmission path of the other device. A first subtraction circuit receives an output of the memory and an output of the coupling estimation block. In a preferred embodiment, the device also includes an echo estimation block having a finite impulse response adaptive filter that receives the transmission signal and an error signal derived from the reception path, and a second subtraction circuit that receives an output of the memory and an output of the echo estimation block.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.